1. Field of the Invention
The present invention relates to an in-plane switching type liquid crystal display device and manufacturing method thereof. More specifically, the present invention relates to material of a spacers for keeping a space between panels and a method of distributing the spacers.
2. Description of the Background Art
A liquid crystal display device which advantageously has the feature of thin form, light weight, low power consumption and so on has been widely used as a display device for a watch, a pocket calculator and the like. Especially, a TN (twisted nematic) type liquid crystal display device of which active driving is implemented by a thin film transistor (TFT) or the like has been increasingly popular in the field of display devices for word processors, personal computers and the like for which CRT (cathode lay tube) has been conventionally used. The TN type liquid crystal display device, however, generally has a narrow angle of visibility, and when viewed obliquely, contrast decreases and gradation is inverted. In order to solve this problem, an in-plane switching type liquid crystal display device has been proposed.
The principle of operation of the in-plane switching type liquid crystal display device will be described in the following with reference to FIGS. 3A and 3B. FIGS. 3A and 3B are schematic illustrations showing the state of orientation of liquid crystals in a common in-plane switching type liquid crystal display device. In the figures, interdigital electrode 1a and 1b represent a plurality of electrodes arranged parallel to each other on the same substrate. Interdigital electrode 1a and 1b are formed on electrode substrate 3. Between a counter substrate 4 and electrode substrate 3, liquid crystal molecules 2 are arranged with an orientation film (not shown) interposed. An equipotential line 5 of an electric field is generated by a voltage applied between interdigital electrode 1a and 1b. A pair of polarization plates 7, 8 have transmission axes in the directions of the arrows. FIG. 3A shows the state of orientation of liquid crystal molecules 2 when no voltage is applied between the pair of interdigital electrode 1a and 1b, and FIG. 3B shows the state of orientation when a voltage is applied between the interdigital electrode 1a and 1b. When the voltage is not applied, liquid crystal molecules 2 are oriented in the direction 10 shown in FIG. 3A. At this time, when the polarization plate 7 is arranged with the transmission axis thereof aligned with the direction of orientation 10 and polarization plate 8 is arranged orthogonal thereto, an incident light beam 6 cannot pass through polarization plate 8, and therefore the display is black (dark). When a voltage is applied between interdigital electrode 1a and 1b, an electric field is generated approximately parallel to the substrate surface, and the direction of orientation of liquid crystal molecules 2 changes as shown in FIG. 3B. More specifically, birefringence of the liquid crystal layer changes, so that the incident light beam passes through polarization plate 8 and goes out as outgoing transmission light beam 9, resulting in white (bright) display. In the in-plane switching type liquid crystal display device, the liquid crystal molecules 2 switch approximately parallel to the substrate surface dependent on application/non-application of the voltage. Therefore, even when the viewing direction changes, optical contribution of liquid crystal molecules 2 hardly changes. Therefore, even when viewing angle changes, the contrast ratio and display quality are not degraded. Thus, the device provides excellent viewing angle characteristics.
In an actual in-plane switching type liquid crystal display device, uniaxial orientation disorder occurs near a spacer or spacers used for keeping a space between panels, causing birefringence with respect to the incident light beam, resulting in elliptic polarization. This allows passage of light beam through the other polarization plate 8, which passage of light is viewed as whitening in the black (dark) state. The state of light passage will be described with reference to FIGS. 4A and 4B. In the figure, there is a portion 11 where liquid crystal orientation is in disorder. A conventional spacer 12 is of divinyl benzene, acrylate resin or the like. Charges 13 are held by spacer 12. In the figure, portions coirresponding to those of FIGS. 3A and 3B are denoted by the same reference characters.
The conventional spacer 12 has such a nature that tends to hold charges on its surface during the process of manufacturing a liquid crystal display device. When the conventional spacer 12 is distributed to a surface of either one of electrode substrate 3 or counter substrate 4 on which an orientation film (not shown) is formed, spacer 12 is charged to positive or negative polarity, through contact between spacer 12 and a carrier pipe system of a spacer distributing apparatus. Conventionally, the carrier pipe system of the spacer distributing apparatus is generally formed of a resin based material such as Teflon (trademark) or stainless steel, in view of cost and ease in molding. As spacer 12 is brought into contact with the carrier pipe system formed of a material different from that of spacer 12, polarization occurs near the contact surface, and when the contact surface is separated, charges 13 are generated on the surface of spacer 12. Such a phenomenon is referred to as separation charging. When a liquid crystal material is introduced in this state, liquid crystal molecules 2 in the vicinity of spacer 12 come to be in the state where monoaxial orientation is disordered, as shown in FIG. 4A or 4B, dependent on the state of generation of charges 13 held by spacer 12 or on the characteristics of the liquid crystal material. Here, at the portion 11 where liquid crystal orientation is in disorder, light beam entering from the lower side of the panel is transmitted to the upper side of the panel because of the influence of birefringence of the liquid crystal layer, and viewed as a light passage (whitening). This light passage is particularly noticed which display is in black (dark) state. Contrast ratio, which is one of the display characteristics of the liquid crystal display device, that is, (brightness (transmittance) in the white (blight) state)/(brightness (transmittance) in the black (dark) state) degrades, as brightness (transmittance) in the black state increases. Further, by this light passage, the viewer has an impression that display screen is rough.
In order to provide a liquid crystal display device presenting images of uniform high quality without light passage, Japanese Patent Laying-Open No. 9-160051 proposes a method in which the spacers for defining a space between a pair of electrode substrate are directly fixed on a prescribed positions of the orientation film formed on the electrode substrate, for example on positions of electrode region or on a light shielding film region of the other orientation film. According to this method, the spacers can be distributed and fixed with a narrow pitch and high density only on that orientation film positions which correspond to non-open positions of the liquid crystal display device. Therefore, it is expected that a liquid crystal display device of high quality free of unwanted light leakage, unsatisfactory display or dependency of angle can be obtained. In order to selectively arrange the spacers at prescribed positions, however, it is necessary to add new process steps to the conventional steps of manufacturing. According to the aforementioned Japanese Patent Laying-Open No. 9-160051, it is necessary to newly add the step of irradiating a thermosetting material or the orientation film formed of a thermosetting material, having the function of fixing the spacer material when irradiated with light beam or heat, with ultraviolet ray using a mask pattern, and the step of removing unnecessary spacers not fixed on the orientation film. The increase in manufacturing steps is not desired in view of higher panel cost and lower production yield. Therefore, a method of manufacturing a high quality liquid crystal display device in a simple manner at low cost has been desired.
The present invention was made to solve the above described problems, and its object is to provide a liquid crystal display device having high display quality with high contrast ratio and free of any impression of rough display surface, and to provide the method of manufacturing the same.
The present invention provides an in-plane switching type liquid crystal display device including an electrode substrate having a plurality of electrodes arranged parallel to each other, a counter substrate sandwiching, together with the electrode substrate, liquid crystal with an orientation film interposed, a pair of polarization plates arranged on outer surfaces of the electrode substrate and the counter substrate with respective transmission axes orthogonally crossing with each other, and spacers for keeping constant a space between the electrode substrate and the counter substrate in which a voltage is applied between the plurality of electrodes to generate an electric field approximately parallel to the substrate surface, causing switch of the liquid crystal molecules approximately parallel to substrate surface, and birefringence of the liquid crystal layer changes, whereby a white (blight) display is given when a voltage is applied to the liquid crystal layer and a black (dark) display is given when a voltage is not applied to the liquid crystal layer, characterized in that amount of charges on the spacer surface is made zero or almost zero, whereby difference in brightness in the black (dark) display state between a region where the spacers are arranged and another region where the spacers are not arranged is adjusted to be within a prescribed range.
Assuming that the spacers each have the diameter of 2 to 6 xcexcm and spacer distribution density is 100 to 600 particles/mm2, in the black (dark) display state, brightness in a region where the spacers are arranged is at most twice the brightness of the region where the spacers are not arranged.
The spacer has at least its surface formed of a conductive material.
Further, specific resistance of the surface of the spacer is at most 10 xcexa9xc2x7m.
According to the present invention, the method of manufacturing the liquid crystal display device, which has liquid crystal arranged between an electrode substrate having a plurality of electrodes arranged parallel to each other and a counter substrate with an orientation film interposed, and spacers provided for keeping constant a space between the electrode substrate and the counter substrate, includes the step of distributing the spacers to either one of the electrode substrate and the counter substrate on which the orientation film is formed by using a spacer distributing apparatus having a spacer carrier pipe system formed of the same material as the material of the spacer surface.
Further, the present invention provides a method of manufacturing a liquid crystal display device having liquid crystal arranged between an electrode substrate having a plurality of electrodes arranged parallel to each other and a counter substrate with an orientation film interposed, and spacers provided for keeping constant a space between the electrode substrate and the counter substrate, including the steps of supplying ions of which polarity is opposite to that of charges held by the spacers, to the spacers in the carrier pipe system of the spacer distributing apparatus or to the spacer emitted from the distributing apparatus so as to neutralize the charges on the spacer surface, while distributing the spacer to either one of the electrode substrate or the counter substrate on which the orientation film is formed.
Further, the present invention provides a method of manufacturing a liquid crystal display device having liquid crystal arranged between an electrode substrate having a plurality of electrodes arranged parallel to each other and a counter substrate with an orientation film interposed, and spacers provided for keeping constant a space between the electrode substrate and the counter substrate, including the step of, after the spacers are distributed to either one of the electrode substrate and counter substrate on which the orientation film is formed, supplying ions of which polarity is opposite to that of the charges held by the spacers to the substrate, for neutralizing the charges on the spacer surface.
The ions for neutralizing the charges on the spacer surface may be supplied by blowing air containing ions or by soft X-ray irradiation.
According to the present invention, the amount of charges on the spacer surface can be made zero or almost zero, so that difference in brightness in the black (dark) display state between the region where the spacers are arranged and the region where the spacers are not arranged is adjusted to be within a prescribed range. Therefore, roughness in display is not visually recognized, and thus a liquid crystal display device having high contrast ratio and superior display quality can be obtained.
Further, the device is manufactured through the process including the step of supplying ions of which polarity is opposite to that of the charges on the spacers to the spacers in a carrier pipe system of a spacer distributing apparatus or to the spacers emitted from the distributing apparatus so that the spacers are distributed to the substrate on which the orientation film is formed while the charges on the spacer surface are neutralized. Therefore, it is unnecessary to increase the number of process steps for manufacturing the liquid crystal display device, and therefore it becomes possible to manufacture high quality liquid crystal display device at a low cost with high production yield.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.